Re: [PATCH-tip v4] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()

[Waiman Long <longman@redhat.com>]

On 11/24/22 21:39, Waiman Long wrote:
> In general, a non-null user_cpus_ptr will remain set until the task dies.
> A possible exception to this is the fact that do_set_cpus_allowed()
> will clear a non-null user_cpus_ptr. To allow this possible racing
> condition, we need to check for NULL user_cpus_ptr under the pi_lock
> before duping the user mask.
>
> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
> Signed-off-by: Waiman Long <longman@redhat.com>

This is actually a pre-existing use-after-free bug since commit 
07ec77a1d4e8 ("sched: Allow task CPU affinity to be restricted on 
asymmetric systems"). So it needs to be fixed in the stable release as 
well. Will resend the patch with an additional fixes tag and updated 
commit log.

Cheers,
Longman

> ---
>   kernel/sched/core.c | 32 ++++++++++++++++++++++++++++----
>   1 file changed, 28 insertions(+), 4 deletions(-)
>
>   [v4] Minor comment update
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8df51b08bb38..f2b75faaf71a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2624,19 +2624,43 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
>   int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>   		      int node)
>   {
> +	cpumask_t *user_mask;
>   	unsigned long flags;
>   
> +	/*
> +	 * Always clear dst->user_cpus_ptr first as their user_cpus_ptr's
> +	 * may differ by now due to racing.
> +	 */
> +	dst->user_cpus_ptr = NULL;
> +
> +	/*
> +	 * This check is racy and losing the race is a valid situation.
> +	 * It is not worth the extra overhead of taking the pi_lock on
> +	 * every fork/clone.
> +	 */
>   	if (!src->user_cpus_ptr)
>   		return 0;
>   
> -	dst->user_cpus_ptr = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
> -	if (!dst->user_cpus_ptr)
> +	user_mask = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
> +	if (!user_mask)
>   		return -ENOMEM;
>   
> -	/* Use pi_lock to protect content of user_cpus_ptr */
> +	/*
> +	 * Use pi_lock to protect content of user_cpus_ptr
> +	 *
> +	 * Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent
> +	 * do_set_cpus_allowed().
> +	 */
>   	raw_spin_lock_irqsave(&src->pi_lock, flags);
> -	cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
> +	if (src->user_cpus_ptr) {
> +		swap(dst->user_cpus_ptr, user_mask);
> +		cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
> +	}
>   	raw_spin_unlock_irqrestore(&src->pi_lock, flags);
> +
> +	if (unlikely(user_mask))
> +		kfree(user_mask);
> +
>   	return 0;
>   }
>